In using this type of substrate processing apparatus to form a silicon nitride (Si3N4) film or the like by a method such as a CVD (Chemical Vapor Deposition) method, a method is known that heats a low-temperature portion such as a furnace opening portion to suppress ammonium chloride (NH4Cl) or other by-products from adhering to the low-temperature portion (see Patent Document 1). However, even with this method, adhesion of by-products may still occur in portions of the reaction chamber where the processing gas stagnates, generating particles. As a countermeasure, a method is known that suppresses adhesion of by-products by purging a gas from around the region where the adhesion of by-products occurs and thereby preventing the stagnation of the gas (see Patent Document 2).    Patent document 1: JP-A-8-64532    Patent Document 2: JP-A-10-335317